1. Field of the Invention
The device of the present invention generally relates to nuclear fuel assemblies and, more particularly, to new and improved apparatus and methods for preventing the rotation about their longitudinal axes of elongated rods within nuclear fuel assemblies.
2. Description of the Prior Art
The generation of a large amount of heat energy through nuclear fission in a nuclear reactor is old and now well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of the nuclear fuel rods are grouped together to form separately removable nuclear fuel assemblies. A number of such nuclear fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear fuel rods and as a neutron moderator.
A typical fuel assembly may be formed by a 7.times.7 or an 8.times.8 array of spaced-apart, elongated rods supported between upper and lower tie plates, the rods normally being several feet in length, for example, from eight to fourteen feet in length, being approximately one-half inch in diameter and being typically spaced from one another by a fraction of an inch. In a typical boiling water reactor fuel assembly, the sixty-four rods that form the 8.times.8 array may be either sixty-four fuel rods or one or more non-fueled, water moderator rods with the remaining rods being fuel rods. In order to provide proper coolant flow past the fuel rods to remove the heat therefrom, the fuel rods are typically maintained in a fixed, spaced apart relationship and are restrained from excessive bowing and vibration during operation in the nuclear reactor core by a plurality of spacer grids that are spaced apart vertically along the longitudinal axis of the fuel assembly. Typically, structure is provided within the fuel assembly for maintaining the spacer grids in their respective vertical or axial positions along the longitudinal axis of the fuel assembly. For example, U.S. Pat. Nos. 3,350,275 and 3,466,226 disclose the use of fuel rods, commonly referred to as spacer capture fuel rods, for maintaining the position of the spacer grids within the fuel assembly. U.S. Pat. No. 3,802,995 discloses the use of a non-fueled, water moderator rod, commonly referred to as a spacer capture water rod, for maintaining the axial position of the spacer grids within the fuel assembly.
Typically, the spacer capture rod, whether a fuel rod or a water moderator rod, includes one or more projecting portions formed thereon and located at each spacer grid axial location for engagement with or for limiting the movement of the spacer grids at their respective axial locations. Such projections are configured to pass through one cell of each spacer grid, typically located in or near the center of the spacer grid, when the spacer capture rod is positioned at a first rotational orientation with respect to the longitudinal axis of the spacer capture rod. Subsequently, by rotating the spacer capture rod about its longitudinal axis through a fixed angle, for example, an angle of 45.degree., the projecting portions of the spacer capture rod will engage or coact with the spacer grids at each axial location to prevent or limit the axial movement of the spacer grids. See, as an example, FIGS. 8-12 of the above-mentioned '995 patent. Thus, it becomes important to prevent the subsequent rotation of the spacer capture rod about its longitudinal axis in order to prevent the disengagement of the projecting portions of the spacer capture rod from the spacer grids.
The lower end plug of the spacer capture rod disclosed in the above-mentioned '995 patent includes an extension with a square cross section adapted to fit in a special cavity of a complementary or mating shape in the lower tie plate of the fuel assembly to prevent the rotation of the spacer capture rod. Such an "in-cavity" anti-rotation scheme entails its own set of problems, for example, the possible necessity for accurately machining the inner walls of a sleeve in the tie plate, that may be undesirable for many applications.